BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gas generating agent used in an air bag apparatus for automobiles, a propelling apparatus using a gas, and a gas generating apparatus applied to high pressure installations and the like, more particularly, to a gas generating agent which provides little generation of an unfavorable gas, chemically stable, exhibits excellent gas generation efficiency, and can generate a gas at high speed.
2. Description of the Related Art
Solid gas generating agents are provided as a mixture of a fuel and an oxidizer or as a material having a fuel component and an oxidizer component together in its molecule. The proposed gas generating agents are used in an air bag apparatus for automobiles, a propelling apparatus utilizing a gas such as a rocket and the like, a pressure generating apparatus driving an automatic control apparatus, and the like.
These apparatuses are intensely required to be light and portable. Depending on use environments, it is expected that they could be exposed to high temperature and strong impact. Further, as a result of use, generation of an unfavorable gas should be particularly avoided. Therefore, gas generating agents used in these apparatuses are required to have properties that generation of an unfavorable gas is suppressed, the gas generation amount per unit weight or per unit volume is large, the gas generation speed is high, chemical stability against heat and impact is obtained, and the like.
For example, the unfavorable gas are nitrogen oxides, halogenated gases, carbon monoxide, high concentration carbon dioxide and so on. For suppression of generation of these gases, azide as main component has been proposed for air bag. However, sodium azide is unfavorable itself, and handling thereof needs particular attention.
In a propelling apparatus applied to a rocket, addition of ammonium perchlorate as an oxidizer is proposed. With this gas generating agent a hydrogen chloride gas is generated after burning. This gas possibly causes acid rain, and from the standpoint of care to environment, a gas generating agent containing no halogen is recently required.
As the gas generating agent containing no halogen, arts disclosed in WO95/19944 and WO96/20147 have been proposed. WO95/19944 discloses. as a gas generating agent, a complex salt composed of a ligand containing nitrogen, the coordination center made of a transition metal cation, and an anion containing oxygen. WO96/20147 discloses a compound Mg 3(H3NCONH3) 2(NO3) as a fuel component.
SUMMARY OF THE INVENTION
The gas generating agent disclosed by WO95/19944 is not approved as sufficient in the point of the generation amount of a gas per unit weight. The gas generation efficiency is calculated below, regarding the weight of a gas generated per unit weight as the gas generation efficiency. The reaction of generation of a gas from the gas generating agent disclosed by WO95/19944 is represented by the following reaction formula.
Since 3 mol of water (molecular weight: 18) and 2 mol of nitrogen (molecular weight: 28) are generated as gases, from 1 mol of a gas generating agent having a molecular weight of 189, the gas generation efficiency is not more than about 60%.
In the case of the gas generating agent disclosed by WO96/20147, when KNO3 is applied as an oxidizer, the reaction formula is represented by the following formula.
The gas generation efficiency is likewise calculated to be about 72%, enhanced than that in the above proposed art.
However, in any of the above proposed arts, when a gas generating apparatus is operated at a low temperature of about −40° C. For example, it is observed that the volume of the generated gas is reduced. For example, when applied to an air bag for automobiles, sufficient inflation force may not be obtained.
The present invention has been accomplished to solve these problems of the proposed gas generating agents. The object thereof is to provide a gas generating agent which generates little unfavorable gas, stable at high temperature, exhibits excellent gas generation efficiency, and provides a tendency that the volume of a generation gas does not decrease easily even at lower temperature.
The inventor has investigated a cause for the phenomenon of reduction in the volume of a generated gas. In the case of the gas generating agents disclosed in WO95/19944and WO96/20147, the proportions of water molecules occupying in the generated gas are 60% and about 52% by molar ratio, respectively. It was found that when the gas generating apparatus is operated at lower temperature, a large amount of these water molecules become quickly to liquid water, and accordingly, the volume of a gas decreases.
As the oxidizer causing little generation of an unfavorable gas, nitrate compounds such as ammonium nitrate and metal nitrate may be applied. However, ammonium nitrate has low burning speed Further, since the metal nitrates contain a large amount of water of crystallization, they tend to be melted at higher temperature, together with the gas volume reduction at low temperature as described above.
The present invention has been accomplished in view of the above-mentioned investigations, and the present invention is composed of a double salt represented by the chemical formula:
The above-mentioned M is usually a metal element forming a nitrate salt containing water of crystallization, the above-mentioned i is a numerical value corresponding to the valency of the metal element M, and the above-mentioned n is a molar number between 1 and 20.
M is preferably one or more metal elements selected from the group consisting of copper, magnesium, calcium, aluminum, nickel, cobalt and zinc.
The present invention is characterized in that a metal nitrate of M(NO3)1 is made into a double salt with ammonium nitrate, to exclude water of crystallization usually contained in a metal nitrate. A gas generating agent which generates little unfavorable gas, stable at high temperature, exhibits high gas generation efficiency, and provides a tendency that the volume of a generation gas does not decrease easily even at lower temperature has been actualized by addition of the above-mentioned double salt as an oxidizer to a gas generating agent.
DETAILED DESCRIPTION OF THE INVENTION
As the stable metal nitrate containing water of crystallization, nitrate salts of copper, beryllium, magnesium, mercury, zirconium, bismuth, chromium, manganese, iron, nickel, cobalt and the like are exemplified. Preferable metal nitrates, which exhibit little metal toxicity and relatively cheap, are nitrate salts of copper, magnesium. calcium, aluminum, nickel, cobalt and zinc. Further, by use of a nitrate salt of magnesium and copper an oxidizer having further lower toxicity and very stable is obtained.
A suitable amount of ammonium nitrate is mixed per mol of the above-mentioned metal nitrate salt, to give a double salt or a complex salt. When the amount of ammonium nitrate is less than 1 mol, the gas generation efficiency decreases, and when over 20 mol, burning speed lowers . Therefore, ammonium nitrate in an amount of 1 to 20 mol, preferably 2 to 16 mol, further preferably 2 to 9 mol are mixed, to form a double salt or a complex salt.
When the above-mentioned ingredients are mixed, water of crystallization is substituted by ammonium nitrate and start to be liberated. When this is dried, liberation of water of crystallization of approximately the theoretical amount is completed, giving formation of solid. An oxidizer obtained as such does not contain water of crystallization, thereby it is stable even at higher temperature and is so oxidative as to increase the gas generation efficiency. Thus, such an oxidizer is suitable as a solid gas generating agent.
According to the present invention, two or more of the double salts having distinctive n values can be combined, and two or more of the double salts having distinctive M values can also be combined. Any of the double salts of the present invention is stable up to 200° C., and solid up to 120° C. Copper nitrate containing water of crystallization is melted at 114° C. and decomposed at 170° C., while the double salt of the present invention has excellent chemical stability at higher temperature.
Depending on the kind of M, or as single M can have different number of water of crystallization, the amount of water of crystallization contained in a metal nitrate varies. The amount of water of crystallization may not be particularly restricted. Those stable under synthesis environments can be used, for example, in the case of copper nitrate, a hexahydrate is stable at relatively lower temperature of 26° C. or lower, and a trihydrate is stable at relatively higher temperature of over 26° C.
When these metal nitrates and ammonium nitrate are mixed, water may be released to give viscous condition or slurry condition, or severe moisture absorption leading to poor handling, in some cases. In these cases, water may be added to prepare a solution to be mixed. This mixture is dried, and the drying conditions can be determined depending on the kind thereof. For example, if drying is effected only by heating, the heating temperature is from 60 to 150° C., preferably from 90 to 130° C. When unstable particularly at higher temperature, the temperature can be lowered by drying in vacuo.
An oxidizer composed of the above-mentioned double salt can be mixed with organic substances or nitrogen-containing organic substances to produce a self-burning solid and a solid gas generating agent. As the organic substances, to which a fuel and the like in general use may be applied, nonvolatile petroleum components, saccharose, sorbic acid and the like are exemplified. It is possible to use generally used polymer compounds such as polybutadiene, polypropylene glycol and the like having a hydroxyl group at the end, thermosetting type polymers having a prepolymer such as azide polymer and a hardening agent as the main components. To these compounds, for example, powders of aluminum, magnesium, boron and zirconium, nitramine compounds, for example, RDX, HMX and the like, can be added, to increase heat generation property to become high energy compound. As the organic substance used in the case of generation of a gas containing a nitrogen gas, there are typically listed compounds containing a triazole or tetrazole ring, for example, bitetrazole, 5-aminotetrazole and the like and guanidine derivatives, for example, aminoquanidine, nitroquianidine, dicyandiamide, guanidine nitrate and the like.
Of the above-mentioned organic substances, tetrazole derivatives are particularly suitable as a gas generating agent for air bags since they generate a large amount of nitrogen, exhibit little generation of CO2 and H2O, and generate a large amount of nitrogen. Of them, 5-aminotetrazole is highly stable and easy to be handled. A mixture of a double salt of copper nitrate and ammonium nitrate of the present invention with 5-aminotetrazole is not melted, even at 126° C. A gas generating agent composed of a mixture of 5-aminotetrazole and ammonium nitrate is melted at 100° C. to 110° C., therefore, the gas generating agent of the present invention is thermally more stable comparatively.
In the double salt of copper nitrate and ammonium nitrate of the present invention, when the amount of ammonium nitrate is in the range from 1 to 20 mol per mol of copper nitrate, heat absortion by a phase transition of ammonium nitrate from rhombic system to tetragonal system at 84° C. and another phase transition from tetragonal system to isometric system at 125° C. As not measured. Namely, It is suggested to be physically stable at higher temperature. When the amount of ammonium nitrate is large than the above-mentioned amount, the above-mentioned phase transition remains. In that case, a stabilizer for ammonium nitrate, for example, potassium nitrate, nickel oxide and the like may also be added for stabilization of phase. The above-mentioned disappearance of the phase transition of ammonium nitrate is a feature commonly recognized in double salts of the present invention including copper nitrate.
The burning speed of gas generating agent composed of the oxidizer of the present invention and organic substances, particularly composed of the double salt of copper nitrate and ammonium nitrate of the present invention and 5-aminotetrazole is higher than that of a composition with an ammonium nitrate single body and organic substance, and suitable for an inflator.